Notional Amounts for Each Category of Derivative Activity (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Derivative
Balance at beginning of period	$ 1,455.3	$ 1,403.7	$ 1,477.4	$ 1,432.8
Additions	268.1	10.0	278.1	114.0
Terminations	21.8	77.6	53.9	210.7
Balance at ending of period	1,701.6	1,336.1	1,701.6	1,336.1
Receive Variable/Pay Fixed
Derivative
Balance at beginning of period	150.0	174.0	150.0	174.0
Additions	0	0	0	0
Terminations	0	24.0	0	24.0
Balance at ending of period	150.0	150.0	150.0	150.0
Receive Fixed/Pay Fixed
Derivative
Balance at beginning of period	608.3	474.7	630.4	508.8
Additions	250.1	10.0	250.1	10.0
Terminations	3.8	3.6	25.9	37.7
Balance at ending of period	854.6	481.1	854.6	481.1
Receive Fixed/Pay Variable
Derivative
Balance at beginning of period	600.0	675.0	600.0	750.0
Additions	0	0	0	0
Terminations	0	40.0	0	115.0
Balance at ending of period	600.0	635.0	600.0	635.0
Credit Default Swaps
Derivative
Balance at beginning of period	97.0	70.0	97.0	0
Additions	0	0	0	70.0
Terminations	0	0	0	0
Balance at ending of period	97.0	70.0	97.0	70.0
Forwards
Derivative
Balance at beginning of period	0	0	0	0
Additions	18.0	0	28.0	24.0
Terminations	18.0	0	28.0	24.0
Balance at ending of period	0	0	0	0
Options Held
Derivative
Balance at beginning of period	0	10.0	0	0
Additions	0	0	0	10.0
Terminations	0	10.0	0	10.0
Balance at ending of period	$ 0	$ 0	$ 0	$ 0